Tiggermoth,
You do the Veedub engine a grave injustice! It is a bl@@dy marvelous little engine and very reliable. It's biggest problem is people's over estimation of how much power it can put out. Even the biggest VW aircooled engine is only good for about 45hp of sustained power
You can increase capacity, but any extra power should be regarded as extra take off and climb power only. If you bear this in mind and choose to use the VW in a suitable aircraft, you'll have no problem. The reason is simply one of cooling
The finned area of the cylinders (and more importantly) the heads is the same for a 2050cc as it is for a 1600, which in turn is little more than the original 1200! Unless you go for the watercooled heads then it's just a matter of physics that the engine WILL NOT be able to cool itself as power is increased. Therefore you find that you'll have to throttle back and back or risk overheating the heads and bu@@ering the valves
They do like their valves adjusting regularly (between 30-50 hours) but it's a 20 minute job so no real problem. Carb ice is really only a problem with bottom slung carbs or poorly designed hot air systems. I've had lot worse troubles with carb ice in Continental's than VW's
In an aircraft such as a Jodel D9, Turb or my T31, the demand on power is low and the VW is well matched and proven to be reliable. The VP1 and worse the VP2 are very draggy so demand for power is high so the VW is at a disadvantage straight away. The VP1 should be able to fly happily on 45hp and does, but a little extra for take off is ideal. For that reason an 1834 is much better in a VP1 than a 1600, which is really underpowered for such a draggy airframe. The VW has also proven ok...ish in a few two seaters, such as the Falke and Jodel D18, but again these airframes are a bit more slick and usually have well designed pressure cowls.
The VW in my Slingsby T31m is the same engine that powered my VP2, but you'd think it was a totally different engine! In the VP2 it would get you off the ground ok, but as you climbed past 500 you could visibly see the CHT rising, by 1500 you were nearing the redline and had to ease off and cruise climb. On anything but the coldest days, 3000 feet was the highest you could go as any more power applied would cause the temps to climb quicker than the aircraft! In the T31 I get well in excess of 500fpm, haven't yet topped out (only been up to 5000' so far though) and the temperatures never seem to rise over the middle range.
To be fair, my old VP2 had a terribly baffled pressure cowl and it did fly safely and surprisingly quickly if you didn't push it. A bit like a speedboat, you could get it on "the step" with a shallow dive to 75knots and then it would stay there, but if you just tried in level flight the best you could accelerate it to was 70. A VP1 with a well baffled 1834 should be a good match and I'd have no worries about the reliability of the VW in it.
SS